This invention relates to an antistatic sheet material used for forming packages or the like for containing electrostatically sensitive components and protecting them from potentially damaging electrostatic charges, to packages formed therefrom, and to a method of making the same.
As electronic components and units have become increasingly more complex, while achieving ever smaller dimensions, the problem of damage to such components due to electrostatic discharge has become of major concern. The buildup of electrostatic charges on the packages for such components can result in the discharge of a spark which can arc over or otherwise damage or destroy the components.
Similarly, the medical and pharmaceutical industries have requirements for ultraclean instruments and drug substances. Packaging of such instruments and drugs is a problem because buildup of electrostatic charges on the packaging materials attracts dust and other contaminants from the air and surroundings. Such contaminants may become mixed with the chemicals of drugs causing purity problems. Likewise, when the package is opened or the product poured out a static charge is generated on the product which may attract contaminates.
Accordingly, a number of packages and packaging materials have been developed which have antistatic properties. These antistatic packages have found wide use in a number of industries including the aerospace, chemical and pharmaceutical, and computer and electronics industries. Such packages are designed to prevent the buildup of electrostatic charges on the product contained therein, and may also be designed to provide protection against external electric fields.
For example, many packages have been fabricated of plastic films or sheets compounded with or coated with antistatic additives such a quaternary amines, amine salts or soaps, polyethylene glycols or ethers, and the like. These agents act as humectants, absorbing moisture from the air, to reduce friction within the package and static charge buildup on the package. However, such antistatic agents may not be permanent (i.e.. may migrate to the surface and be lost from the plastic) and are humidity dependent.
Other packages have included one or more layers of conductive material to form a protective envelope or Faraday cage-type structure about the product. A Faraday cage may be defined as an electrostatic shield composed of a continuous mesh or series of interconnected electrical conductors which surrounds a defined volume of space. Examples of packages utilizing conducting layers include Yenni Jr., et al, U.S. Pat. Nos. 4,154,344 and 4,156,751; Petcavich, U.S. Pat. No. 4,424,900; Horii, U.S. Pat. No. 4,407,872 and Dedow, U.S. Pat. Nos. 4,471,872 and 4,496,406. See also BraZier, U.S. Pat. No. 4,576,649 which discloses an antistatic laminated plastic package for electrically non-conductive pulverulent material.
Other forms of antistatic laminated plastic packages are known. For example. Richmond Technology, Inc. has in recent years marketed RCAS 3600 Antistatic. RF/EMI Shielding Bags and RCAS 4200 Static Shielding Bags. The RCAS 3600 bags are advertised as having an aluminum foil layer buried between an antistatic treated Tyvek (a registered trademark of DuPont Co.) and Richmond's RCAS 1200 antistatic polyethylene. The RCAS 4200 bags are advertised as having an aluminum metallization layer sandwiched between a non-sparking outer layer of strong polyester and an inner layer of RCAS 1200 antistatic polyethylene.
Even more recently Neal U.S. Pat. No. 4,648,508, issued Mar. 10, 1987. discloses a method for coextruding a single film construction with two distinctly different surface characteristics for the purpose of an electronic package composite or envelope. The envelope has an inner anti-static polyethylene layer having a surface resistivity of less than 10.sup.12 ohms/sq. and an outer conductive carbon-containing polyethylene layer having a surface resistivity of less than 10.sup.3 ohms/sq. Rayford et al, U.S. Pat. No. 4,738,882, issued Apr. 19. 1988, discloses an antistatic laminated sheet material comprising an insulating layer, a metal layer of surface resistivity not exceeding about 10.sup.4 ohms/sq. and a conductive ink layer.
Finally, reference is made to grandparent application Ser. No. 785,808, which issued as U.S. Pat. No. 4,756,414 on July 12, 1988. The antistatic sheet material and package there disclosed utilizes an electron beam curable antistatic material which is particularly advantageous in rendering antistatic the outer surfaces of the laminated material. However, the preferred process disclosed requires three separate electron beam treatment steps which are time consuming and costly. In addition, some problems in ply delamination, low seam strength, and excessive odor have on occasion been encountered.
Accordingly, the need exists for a more efficient method of producing a laminated antistatic sheet material and a laminated antistatic sheet material and package having improved properties.